Electrochemical Kinetic Investigations of the Reactions of [FeFe

Sep 22, 2009 - Received July 3, 2009; E-mail: [email protected]. Abstract: A major ..... H-domains that house the H-cluster are very simi...
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Electrochemical Kinetic Investigations of the Reactions of [FeFe]-Hydrogenases with Carbon Monoxide and Oxygen: Comparing the Importance of Gas Tunnels and Active-Site Electronic/Redox Effects Gabrielle Goldet,† Caterina Brandmayr,† Sven T. Stripp,‡ Thomas Happe,‡ Christine Cavazza,§ Juan C. Fontecilla-Camps,§ and Fraser A. Armstrong*,† Inorganic Chemistry Laboratory, Department of Chemistry, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom, Ruhr-UniVersität, Lehrstuhl fur Biochemie der Pflanzen, AG Photobiotechnologie, 44780 Bochum, Germany, and Laboratoire de Crystallographie et Crystallographie des Prote´ines, Institut de Biologie Structurale, J.P. Ebel, CEA, CNRS, UniVersite´ Joseph Fourier, 41, rue J. Horrowitz, 38027 Grenoble Cedex 1, France Received July 3, 2009; E-mail: [email protected]

Abstract: A major obstacle for future biohydrogen production is the oxygen sensitivity of [FeFe]hydrogenases, the highly active catalysts produced by bacteria and green algae. The reactions of three representative [FeFe]-hydrogenases with O2 have been studied by protein film electrochemistry under conditions of both H2 oxidation and H2 production, using CO as a complementary probe. The hydrogenases are DdHydAB and CaHydA from the bacteria Desulfovibrio desulfuricans and Clostridium acetobutylicum, and CrHydA1 from the green alga Chlamydomonas reinhardtii. Rates of inactivation depend on the redox state of the active site ‘H-cluster’ and on transport through the protein to reach the pocket in which the H-cluster is housed. In all cases CO reacts much faster than O2. In the model proposed, CaHydA shows the most sluggish gas transport and hence little dependence of inactivation rate on H-cluster state, whereas DdHydAB shows a large dependence on H-cluster state and the least effective barrier to gas transport. All three enzymes show a similar rate of reactivation from CO inhibition, which increases upon illumination: the rate-determining step is thus assigned to cleavage of the labile Fe-CO bond, a reaction likely to be intrinsic to the atomic and electronic state of the H-cluster and less sensitive to the surrounding protein.

Introduction

The increasing need for clean, renewable fuels is stimulating new research on hydrogen (H2) production,1-5 and one promising solution is to exploit microorganisms in ‘H2 farms’. In biology, H2 is evolved by metalloenzymes called hydrogenases, in processes ranging from fermentation to photosynthesis. Hydrogenases are highly efficient enzymessso much so that when attached to an electrode, they are, like platinum, superb electrocatalysts of both H2 oxidation and H2 production, at or close to the reversible potential for the 2H+/H2 couple.6-8 †

University of Oxford. Ruhr-Universitat. § Universite´ Joseph Fourier. (1) Navarro, R. M.; Pena, M. A.; Fierro, J. L. G. Chem. ReV. 2007, 107, 3952–3991. (2) Mertens, R.; Liese, A. Curr. Opin. Biotechnol. 2004, 15, 343–348. (3) Hankamer, B.; Lehr, F.; Rupprecht, J.; Mussgnug, J. H.; Posten, C.; Kruse, O. Physiol. Plant. 2007, 131, 10–21. (4) Liu, X. M.; Ibrahim, S. K.; Tard, C.; Pickett, C. J. Coord. Chem. ReV. 2005, 249, 1641–1652. (5) Melis, A.; Happe, T. Plant Physiol. 2001, 127, 740–748. (6) Vincent, K. A.; Parkin, A.; Armstrong, F. A. Chem. ReV. 2007, 107, 4366–4413. (7) Jones, A. K.; Sillery, E.; Albracht, S. P. J.; Armstrong, F. A. Chem. Commun. 2002, 866–867. (8) Hambourger, M.; Gervaldo, M.; Svedruzic, D.; King, P. W.; Gust, D.; Ghirardi, M.; Moore, A. L.; Moore, T. A. J. Am. Chem. Soc. 2008, 130, 2015–2022. ‡

10.1021/ja905388j CCC: $40.75  2009 American Chemical Society

Of the two main classes, [NiFe]- and [FeFe]-hydrogenases, named according to the metals present in the center at which H2 is activated, the [FeFe]-hydrogenases are considered to be more active in H2 production.9 However, a perceived major disadvantage of [FeFe]-hydrogenases (with respect to [NiFe]hydrogenases) is their higher O2 sensitivity.10 The [NiFe]hydrogenases react rapidly with O2 to give inactive, EPRcharacterized, Ni(III) forms that can be reactivated by reduction: Ni-A (‘unready’) is reactivated very slowly whereas Ni-B (‘ready’) can be reactivated within seconds, hence there is a rapid repair mechanism for hydrogenases that produce only Ni-B.6,11 In contrast, the [FeFe]-hydrogenases appear to undergo irreparable damage when exposed to O2 while in their active state (after reduction).12-14 The incompatibility of O2 with [FeFe]-hydrogenases poses a major limitation to progress in ‘biohydrogen’ production, in particular by modified photosyn(9) Frey, M. ChemBioChem 2002, 3, 153–160. (10) Adams, M. W. W. Biochim. Biophys. Acta 1990, 1020, 115–145. (11) Armstrong, F. A.; Belsey, N. A.; Cracknell, J. A.; Goldet, G.; Parkin, A.; Reisner, E.; Vincent, K. A.; Wait, A. F. Chem. Soc. ReV. 2009, 38, 36–51. (12) Erbes, D. L.; King, D.; Gibbs, M. Plant Physiol. 1979, 63, 1138– 1142. (13) Erbes, D. L.; King, D.; Gibbs, M. Plant Physiol. 1978, 61, 23–23. (14) Vincent, K. A.; Parkin, A.; Lenz, O.; Albracht, S. P. J.; FontecillaCamps, J. C.; Cammack, R.; Friedrich, B.; Armstrong, F. A. J. Am. Chem. Soc. 2005, 127, 18179–18189. J. AM. CHEM. SOC. 2009, 131, 14979–14989

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Structures of H-clusters of the [FeFe]-hydrogenase from Clostridium pasteurianum (CpI) and DesulfoVibrio desulfuricans (DdHydAB) constructed using PyMol. A) CpI H-cluster (PDB code: 3C8Y).23 B) DdHydAB H-cluster.25 The two structures were modeled with different bridgehead atomssO for CpI, and N for DdHydABsbut this distinction is not directly relevant for this paper.

Figure 1.

thesis.15 In green algae the O2-sensitivity of the [FeFe]hydrogenase is the bottleneck for producing H2 from sunlight. Production of H2 is stimulated during sulfur deprivation, conditions under which only 10% of the photosystem II remains active and the system effectively becomes anaerobic.16 Green algae that could express an O2-tolerant [FeFe]-hydrogenase would therefore provide much increased levels of H2 production.15 The buried active site of [FeFe]-hydrogenases is actually a complex 6Fe unit known as the ‘H-cluster’ which contains a [4Fe-4S] subcluster (generally referred to as [4Fe-4S]H) in addition to the di-iron subcluster (2FeH).17 The two independent representations of the H-cluster shown in Figure 1 are directly relevant to the catalytically active states known as Hox and Hred that have been extensively characterized.18,19 General features of the structure are as follows: (a) the [4Fe-4S]H subcluster is linked to one of the Fe atoms of the 2FeH subcluster by a bridging cysteine sulfur (the Fe atoms of 2FeH are thus known as ‘proximal’ (Fep) and ‘distal’ (Fed) with respect to the [4Fe4S]H subcluster);20 (b) both Fed and Fep are coordinated by CO and CN- ligands;17 (c) an unusual SCH2XCH2S dithiolate ligand forms a di-µ-thiolato bridge between Fep and Fed, and although opinions differ as to whether the bridgehead atom X is an O or N atom,21-25 recent investigations with 14N HYSCORE have provided direct evidence that X ) N.21 (15) Ghirardi, M. L.; Posewitz, M. C.; Maness, P.-C.; Dubini, A.; Yu, J.; Seibert, M. Annu. ReV. Plant Biol. 2007, 58, 71–91. (16) Hemschemeier, A.; Fouchard, S.; Cournac, L.; Peltier, G.; Happe, T. Planta 2008, 227, 397–407. (17) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. ReV. 2007, 107, 4273–4303. (18) Lubitz, W.; Reijerse, E.; van Gastel, M. Chem. ReV. 2007, 107, 4331– 4365. (19) De Lacey, A. L.; Fernandez, V. M.; Rousset, M.; Cammack, R. Chem. ReV. 2007, 107, 4304–4330. (20) Nicolet, Y.; Lemon, B. J.; Fontecilla-Camps, J. C.; Peters, J. W. Trends Biochem. Sci. 2000, 25, 138–143. (21) Silakov, A.; Wenk, B.; Reijerse, E.; Lubitz, W. Phys. Chem. Chem. Phys. 2009, 11, 6592–9. 14980

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In the structure of Hox, as determined with the CpI enzyme from Clostridium pasteurianum, Fep is coordinated by one CO and one CN- ligand and shares a bridging CO with Fed.20 In turn, Fed is also coordinated by one CO and one CN- ligand, and an additional binding site is vacant or occupied by an exchangeable O-ligand, most likely a water molecule (Figure 1A). In the structure of the [FeFe]-hydrogenase from DesulfoVibrio desulfuricans, which should be in the Hred form, the bridging CO is replaced by a terminal CO on Fed (Figure 1B).26 Recent EPR spectroscopic investigations favor an oxidation state assignment of [4Fe-4S]2+-Fep(I)Fed(II) for Hox, with some spin density delocalized onto the [4Fe-4S]H subcluster,27 although H-clusters from different enzymes show minor variations in electronic structure.28 The EPR-silent Hred state is assigned as [4Fe-4S]2+-Fe(I)Fe(I) which, if protonated, is formally equivalent to the hydrido species [4Fe-4S]2+Fe(II)Fe(II)-H-.17 As also determined by EPR spectroscopy, exogenous CO, a competitive inhibitor of H2 oxidation, reacts with Hox.29 Crystallographic and infrared spectroscopic studies of Hox-CO further show that binding of CO (which is photolabile) occurs at Fed.30-33 Inactivation by anaerobic oxidants gives rise to a form known as Hoxinact, usually formulated as [4Fe-4S]2+-Fe(II)Fe(II), which can be reactivated upon reductionsa process occurring via an intermediate known as Htrans which has been formulated as [4Fe-4S]+-Fe(II)Fe(II), i.e. with the [4Fe-4S]H subcluster reduced.33 The H-cluster is remarkable among non-macrocycle cofactors because the 2FeH subcluster at which H2 is produced is connected to the protein through just a half-share of a cysteine sulfur: it is very much an organometallic-like compound physically enclosed in protein. Despite these intense studies by crystallography and spectroscopy, numerous aspects of the reactions of [FeFe]-hydrogenases remain unresolved. These aspects include the activation process (there is evidence from electrochemical titrations that a two-electron process is also involved)33 and many details of the mechanism of catalysis in either direction, including the exact function of the [4Fe-4S]H subcluster. The nature and products of the degradation by O2 are only now coming to light,34 and a major issue is whether and how H2 production could be sustainable at all in the presence of O2. This article describes mechanistic investigations, by protein film electrochemistry, of the O2 inactivation kinetics of three representative [FeFe]-hydrogenases. These are: the hydrogenase (22) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853–1858. (23) Pandey, A. S.; Harris, T. V.; Giles, L. J.; Peters, J. W.; Szilagyi, R. K. J. Am. Chem. Soc. 2008, 130, 4533–4540. (24) Barton, B. E.; Olsen, M. T.; Rauchfuss, T. B. J. Am. Chem. Soc. 2008, 130, 16834–5. (25) Nicolet, Y.; De Lacey, A. L.; Vernede, X.; Fernandez, V. M.; Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123, 1596–1601. (26) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C.; Fontecilla-Camps, J. C. Structure 1999, 7, 13–23. (27) Silakov, A.; Reijerse, E. J.; Albracht, S. P. J.; Hatchikian, E. C.; Lubitz, W. J. Am. Chem. Soc. 2007, 129, 11447–11458. (28) Kamp, C.; Silakov, A.; Winkler, M.; Reijerse, E. J.; Lubitz, W.; Happe, T. Biochim. Biophys. Acta 2008, 1777, 410–416. (29) Bennett, B.; Lemon, B. J.; Peters, J. W. Biochemistry 2000, 39, 7455– 7460. (30) Lemon, B. J.; Peters, J. W. J. Am. Chem. Soc. 2000, 122, 3793–3794. (31) Lemon, B. J.; Peters, J. W. Biochemistry 1999, 38, 12969–12973. (32) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, C.; Fernandez, V. M. J. Am. Chem. Soc. 2000, 122, 11232–11233. (33) Roseboom, W.; Lacey, A. L.; Fernandez, V. M.; Hatchikian, E. C.; Albracht, S. P. J. J. Biol. Inorg. Chem. 2006, 11, 102–118. (34) Stripp, S.; Goldet, G.; C., B.; Vincent, K. A.; Armstrong, F. A.; Happe, T. Proc. Natl. Acad. Sci. U.S.A. 2009, in press.

O2 Inactivation Kinetics of Three [FeFe]-Hydrogenases

from a sulfate-reducing bacterium, DesulfoVibrio desulfuricans, abbreviated as DdHydAB which has been crystallographically characterized;26 the hydrogenase from Clostridium acetobutylicum, abbreviated as CaHydA, which is potentially of importance for H2 production by anaerobic fermentation and has high sequence similarity with the crystallographically characterized CpI hydrogenase from C. pasteurianum,22 and the hydrogenase known as CrHydA1 from the green alga Chlamydomonas reinhardtii, which is of interest for photosynthetic H2 production.35 Both DdHydAB and CaHydA contain a series of Fe-S clusters17 to relay electrons within the protein for transfer to and from the redox partner (in our case the electrode); in contrast CrHydA1 possesses no Fe-S clusters apart from the [4Fe-4S]H subcluster,36,37 but it is nonetheless electrocatalytically active when adsorbed on an electrode.34 Although the three enzymes differ in their overall tertiary and quaternary structures, their H-domains that house the H-cluster are very similar.17 In protein film electrochemistry, an enzyme is immobilized on the surface of an electrode such that its properties are controlled directly by the electrode potential.6,38 Catalytic activity in either direction, oxidation or reduction, can be driven and recorded at any particular potential value and the catalytic rate is directly proportional to the current that flows. Various gas mixtures (produced by mass-flow controllers) can be introduced and flushed from the sealed electrochemical cell in which the electrode is rotated rapidly to provide precise hydrodynamic control (supply of reactants and removal of products). A particular advantage of this approach is that turnover activity in either direction is immediately and directly observed from the catalytic current; thus, rates of change of activity, such as those induced by CO or O2, are extracted directly from the variation of current with time, all as a precise function of the electrode potential. In this way, extremely complex reactivities become resolvable; therefore, this technique both complements and instigates structural and spectroscopic investigations. We first establish, for each enzyme, how H2 oxidation and H2 production are affected by the concentrations of H2 and CO; next we examine the kinetics of CO binding and release and correlate these data with equilibrium values; we then examine the kinetics of O2 inactivation of H2 oxidation activity; finally, we exploit CO inhibition as a tool to investigate H2 production in the presence of O2. Recent studies have established that CO is able to protect [FeFe]-hydrogenases against inactivation by O2,34,39 an observation suggesting that the sequence of destruction is initiated by O2 coordinating to the same site at which exogenous CO binds, i.e. Fed.30-33 Thereafter, the mechanism remains less clear, but recent EXAFS evidence obtained with CrHydA1 shows that the [4Fe-4S]H subcluster is altered more than the 2FeH subcluster.34 Our experiments show clearly how the destructive power of O2 varies among the hydrogenases with an interesting dependence on catalytic direction (H2 oxidation compared to H2 production)sthus implicating sensitivity to the oxidation level of the active site. The results provide insight for the quest for solutions to the oxygen problem in biohydrogen (35) Ghirardi, M. L.; Dubini, A.; Yu, J. P.; Maness, P. C. Chem. Soc. ReV. 2009, 38, 52–61. (36) Stripp, S.; Sanganas, O.; Happe, T.; Haumann, M. Biochemistry 2009, 48, 5042–5049. (37) Happe, T.; Naber, J. D. Eur. J. Biochem. 1993, 214, 475–481. (38) Le´ger, C.; Bertrand, P. Chem. ReV. 2008, 108, 2379–2438. (39) Baffert, C.; Demuez, M.; Cournac, L.; Burlat, B.; Guigliarelli, B.; Bertrand, P.; Girbal, L.; Leger, C. Angew. Chem., Int. Ed. 2008, 47, 2052–2054.

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production by photosynthesis. The functional differences between the [FeFe]-hydrogenases are not immediately evident from the structures that have been obtained for DdHydAB and Cp1. Methods Previously reported protocols were employed to obtain pure samples of DdHydAB,40 CrHydA1 and CaHydA.41 In each case levels of CO, O2 and H2 present during cell growth were extremely low. Protein film electrochemistry experiments were carried out in an anaerobic glovebox (M Braun) comprising a N2 atmosphere. The solutions contained 0.05 M phosphate buffer with 0.10 M NaCl as additional supporting electrolyte, and were prepared using standard reagents NaCl, NaH2PO4 and Na2HPO4 (Analytical Reagent grade, Sigma) in purified water (Millipore 18 MΩ cm). The working electrode was a disk (area 0.03 cm2) of pyrolytic graphite oriented so that the ‘edge plane’ faced the solution. This is called a pyrolytic graphite edge (PGE) electrode and it was used in conjunction with an electrode rotator (EcoChemie) that fitted snugly into a specially designed, gastight, glass electrochemical cell. The cell featured a water jacket for temperature control. Due to the light sensitivity of [FeFe]-hydrogenases the entire surface of the cell was blacked-out with adhesive masking tape. In experiments to detect photolabilization, the tape at the bottom of the cell was removed to allow illumination from a 150 W lamp placed just beneath the cell (see Figure SI.1, Supporting Information). A saturated calomel reference electrode (SCE) was placed in a side arm containing 0.1 M NaCl, connected to the main cell compartment by a Luggin capillary. A Pt wire was used as the counter electrode. Potentials (E) are quoted with respect to the standard hydrogen electrode (SHE) using the correction ESHE ) ESCE + 242 mV at 298 K.42 Electrochemical experiments were performed using an electrochemical analyzer (Autolab PGSTAT10 or 20) controlled by a computer operating GPES software (EcoChemie). Mass flow controllers (Smart-Trak Series 100, Sierra Instruments, U.S.A.) were used to prepare precise gas mixtures (headspace fractions accurate to within 1%) and to impose constant gas flow rates into the electrochemical cell during experiments. Gases used were H2 (Premier grade, Air Products), O2 (Air Products), CO (Research grade, BOC), 1% CO in N2 (Research grade, BOC), N2 (Oxygen-free, BOC) or mixtures of these gases. Henry’s law was applied to estimate the concentration of gas in solution in each experiment.43 Values taken for use at 10 °C were: 100% CO, 1248 µM; 100% O2, 1624 µM. To prepare each enzyme film, the PGE electrode was first polished for 10 s with an aqueous slurry of R-alumina (1 µm, Buehler) and sonicated for 5 s in purified water, before enzyme solution (1.5 µL, containing 0.015-0.15 µg, pH 8) was applied and removed after a few minutes. The electrode was then placed in enzyme-free buffered electrolyte so that the only enzyme molecules being addressed were on the electrode and subjected to the same regime of strict potential control. In all experiments the electrode was rotated at a constant high rate (3000-5000 rpm) to ensure efficient supply of substrate and removal of product. In experiments measuring the effect of O2 on hydrogenasecatalyzed H2 production, it was necessary to estimate the concentration of O2 that the enzyme molecules at the electrode surface actually experience, given that a portion of the total dissolved O2 is consumed by the electrode at this potential (-0.4 V). Using a (40) Hatchikian, C.; Forget, N.; Fernandez, V. M.; Williams, R.; Cammack, R. Eur. J. Biochem. 1992, 209, 357–365. (41) von Abendroth, G.; Stripp, S.; Silakov, A.; Croux, C.; Soucaille, P.; Girbal, L.; Happe, T. Int. J. Hydrogen Energy 2008, 33, 6076–6081. (42) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (43) Sander, R. http://www.henrys-law.org, 1999. J. AM. CHEM. SOC.

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Figure 2. Determination of KMH2 (A and B) and KIapp (C and D) for CaHydA. (Panel A) chronoamperometric experiment performed at -0.05 V, at pH 6, 10 °C. The broken line traces the progression of film loss throughout the experiment. (Panel B) Lineweaver-Burk plot from experiment shown in A fitted to a straight line. (Panel C) chronoamperometric experiment performed to determine KIapp at -0.4 V, at pH 6.0, 10 °C. The concentration of H2 in N2 (%) in the headspace of the cell is indicated. The rotation rate was varied from 3000 to 4000 and 5000 rpm at each concentration of H2 to ensure that inhibition was not mass-transport limited. (Panel D) Plot according to the procedure described by Le´ger et al.45 from which KIapp is determined, showing the line of best fit.

previously published method,44 it was estimated that 0.7% O2 (11 µM in solution at 10 °C) survives to be experienced by the enzyme when 1% O2 is flowed through the headspace of the cell and 4.5% (73 µM) survives when 5% O2 is used. Results Measurements of the H2 Concentration Dependencies for H2 Oxidation and Production. We first evaluated the affinity

of the enzymes for H2 both in terms of KM for H2 as the substrate in H2 oxidation (KMH2) and the apparent inhibition constant KIapp for H2 as the product inhibitor of H+ reduction. Figure 2 shows experiments carried out for CaHydA: analogous experiments were carried out for CrHydA1, but films of DdHydAB were not sufficiently stable to obtain accurate measurements over the period of time required (30-60 min.). Experiments to determine KMH2 were conducted by varying the ratio of H2 to N2 in the headgas and measuring the oxidation current after allowing time for the gas mixture to equilibrate with the cell solution at each concentration of H2 (Figure 2A). The experiments were performed at an electrode potential of -0.05 V to avoid the anaerobic inactivation that occurs at higher potential (see Figure 3). We and others, have noted that KM and KI are potential-dependent quantities,44,45 and the potential must therefore be specified. It was also important to make measurements under conditions where the current was limited by the catalytic rate of the enzyme rather than by mass transport of substrate to (or product from) the electrode. The experiments were therefore carried out at low temperature (10 °C) to decrease the rate of catalysis. The low temperature also minimized the (44) Goldet, G.; Wait, A. F.; Cracknell, J. A.; Vincent, K. A.; Ludwig, M.; Lenz, O.; Friedrich, B.; Armstrong, F. A. J. Am. Chem. Soc. 2008, 130, 11106–11113. (45) Le´ger, C.; Dementin, S.; Bertrand, P.; Rousset, M.; Guigliarelli, B. J. Am. Chem. Soc. 2004, 126, 12162–12172. 14982

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Figure 3. Cyclic voltammograms showing bidirectional electrocatalytic H+ reduction and H2 oxidation by CrHydA1 (A), CaHydA (C) and DdHydAB (E) at pH 6.0 under 1 bar H2. Panels B, D, F show the timedependent voltammograms recorded following introduction of CO, introduced prior to the scans shown here, as the potential was being cycled between -0.55 V and +0.3 V, as it is removed from the cell: these reveal different reactions as CO dissociates and rebinds as a function of potential. Experimental conditions for CrHydA1: 10 °C, scan rate 20 mV/s, inhibition was achieved by injection of CO-saturated buffer to give an immediate concentration of 100 µM CO in solution 5 min prior to the scan shown in panel B. Experimental conditions for CaHydA: 32 °C, electrode rotation 3000 rpm, scan rate 20 mV/s, inhibition was achieved by flushing 100% CO through the cell 5 min prior to the scan shown in panel D. Experimental conditions for DdHydAB: 10 °C, electrode rotation 3000 rpm, scan rate 10 mV/s, inhibition was achieved by injection of CO-saturated buffer to give an instant concentration of 30 µM CO in solution 5 min prior to the scan shown in panel F. The dashed arrows indicate the direction of scanning. The asterisks (*) indicate the potential above which anaerobic inactivation takes place for CrHydA1 and DdHydAB.

rate of film loss. At each H2 concentration the electrode rotation rate was stepped between 3000 and 5000 rpm to ensure that the current (and thus the rate of reaction) was independent of rotation rate. Values of KMH2 were calculated from the x-intercept of the Lineweaver-Burk plot shown in Figure 2B and are summarized in Table 1. Experiments to measure the apparent + inhibition constant (KIapp, defined in eq 1 where KMH is the KM Table 1. Values of KMH2 and KIapp for CrHydA1 and CaHydA Determined at pH 6, 10 °C from the Experiments Shown in Figure 2a

a

enzyme

KMH2/mM

KIapp/ mM

CrHydA1 CaHydA

0.19 ( 0.03 0.46 ( 0.06

3.7 ( 0.6 6.2 ( 1.1

KMH2 and KIapp were calculated at -0.05 and -0.4 V, respectively.

O2 Inactivation Kinetics of Three [FeFe]-Hydrogenases

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for binding of the substrate H+ and KI is the real inhibition constant) were conducted at -0.4 V vs SHE using a similar experimental method (Figure 2C). Values of KIapp were calculated by adapting the method based on that reported by Le´ger et al.45 which requires plotting the data according to eq 2 (in which iN2 is the current recorded under 100% N2 and iH2 is the current recorded at each concentration of H2) as shown in Figure 2D. The KIapp values are included in Table 1. KIapp

)

KI[H+] +

H KM

(

+

1+

(iN2 /iH2) - 1 )

H KM

[H+]

[H2] KIapp

)

(1)

(2)

The KMH2 values are much higher than those we have measured recently for some O2-tolerant [NiFe]-hydrogenases46 and they suggest that the catalytic current for H2 oxidation will be sensitive to changes in H2 levels even close to 1 bar partial pressure (this is particularly so for CaHydA). The results showed that headspace H2 levels should be maintained constant in quantitative H2 oxidation experiments. In contrast, the KIapp values are so high that the presence of H2 is not expected to pose a problem in studies of H2 production. The order of magnitude difference between KIapp and KMH2 suggests immediately that the electronic/catalytic state of the H-cluster exerts a strong effect on binding affinity, with H2 binding more weakly to a more reduced state. Reactions with CO. Cyclic voltammograms of the electrocatalytic activities of DdHydAB, CrHydA1 and CaHydA at pH 6.0 under a flow of 100% H2 are shown in Figure 3 (A, C, Esleft-hand column). These voltammograms show that all the enzymes are bidirectional, with CaHydA being particularly biased in the direction of H2 production. The traces in either potential direction cut through the zero current axis at the cell potential (see vertical line) for the 2H+/H2 couple. At high potentials, all three enzymes undergo inactivation to give a species that is most likely Hoxinact although this process is very slow for CaHydA. The voltammograms in the right-hand column (B, D, F) were recorded during the efflux of CO that had been introduced by injecting a saturated solution (giving a concentration of 100 µM CO in the cell for the experiment on CrHydA1, 30 µM for the experiment on DdHydAB) or flowing 100% CO briefly through the cell (for CaHydA) as the potential was cycled between -0.55 and 0 V prior to the scans shown in panels B, D, and F. Informative changes in the voltammograms of DdHydAB, CrHydA1 and CaHydA occur upon removal of CO during continuous cycling (B, D, F). In all cases the voltammograms show that once the potential is made sufficiently positive to start H2 oxidation, there is an initial increase in current which is followed by a decrease (see the dashed arrow indicating the oxidative sweep). The magnitude of this effect is most apparent after a certain time has elapsed, dependent upon the hydrogenase, and finally the voltammograms resume the expected shapes for H2 oxidation and production at pH 6 under 1 bar H2, analogous to those shown in the left-hand column. This cyclic ‘inhibitor-on/inhibitor-off’ behavior shows that CO binding to the active site is favorable and fast during H2 oxidation but relatively weak during H2 production. Thus CO reinhibits strongly as the potential is raised to oxidize H2, which addresses a more oxidized form of the enzyme (above -0.3 V). The

Figure 4. Inhibition of DdHydAB (red lines), CrHydA1 (blue lines) and

CaHydA (black lines) by injection of 4 µM CO at (A) -0.05 V (H2 oxidation) and (B) -0.4 V (H2 production). Experimental conditions: pH 6.0, 10 °C, 1 bar H2, electrode rotation 3000 rpm. The final level of the current reached upon recovery of H2 oxidation activity of CaHydA is marked by the dashed line in A. The exponential decrease in concentration of dissolved CO is shown by the gray trace (details given in Supporting Information, Figure SI.2).

voltammogram for CrHydA1 also shows an oxidation peak soon after commencing the scan in the negative direction. A similar observation was reported in previous experiments on DdHydAB where it arises from the reactivation of some Hoxinact and its rapid inactivation by CO.47 Further insight into CO binding and release under different conditions is provided by Figure 4. The CO inhibition profiles for DdHydAB, CrHydA1 and CaHydA were obtained for H2 oxidation (Panel A) or H2 production (Panel B) by injecting an aliquot of CO-saturated solution and then recording the catalytic current as the CO is removed by flushing. The same experimental conditions (pH 6.0, 10 °C, 1 bar H2, electrode rotation 3000 rpm) were used for all experiments. In each case, injection of CO-saturated buffer gives an immediate initial CO concentration of 4 µM which decreases exponentially to zero,45 as depicted by the gray trace (right y-axis) which represents the dependence of [CO] on time throughout the experiment; < 0.2 µM CO remains in solution after 500 s. (See Supporting Information (Figure SI.2) for how this dependence is determined.) In the case of CaHydA, inhibition continues to increase even though most of the CO has been flushed out of the cell, indicating that the rate of reaction with CO is slow. For H2 oxidation, the rates and extent of inhibition reached after CO injection decrease in the order DdHydAB > CrHydA1 > CaHydA. The reactivation rates are slow and strikingly similar for all enzymes. For H2 production (panel B) the rates and extent of inhibition by CO again decrease in the order DdHydAB > CrHydA1 > CaHydA. (46) Ludwig, M.; Cracknell, J. A.; Vincent, K. A.; Armstrong, F. A.; Lenz, O. J. Biol. Chem. 2009, 284, 465–477. (47) Parkin, A.; Cavazza, C.; Fontecilla-Camps, J. C.; Armstrong, F. A. J. Am. Chem. Soc. 2006, 128, 16808–16815. J. AM. CHEM. SOC.

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Figure 5. Inhibition by CO and reactivation, observed for H2 oxidation and H2 production with DdHydAB, CrHydA1 and CaHydA. Panels A and B show experiments performed on CaHydA at -0.05 and -0.4 V, respectively, in which the headgas is 80% H2, 20% N2 at the start of the experiment. Carbon monoxide (2%, approximately 25 µM) was introduced by injection of CO-saturated buffer (0.5 mL of buffer saturated with 10% CO, 80% H2, 10% N2) into the 2 mL of buffer already present in the cell and, simultaneously, flushing the headspace with 80%H2, 18% N2, 2% CO for the period of time marked by the gray boxes. Experimental conditions: pH 6.0, electrode rotation 3000 rpm, potentials as indicated. Panels C and D show the dependencies of rates (k′inact) on CO concentration; data are derived from experiments such as those shown in panels A and B, respectively, for DdHydAB, CrHydA1 and CaHydA.

To obtain the kinetics of CO binding for the three [FeFe]hydrogenases it was necessary to maintain a constant CO concentration throughout the time scale of the reaction. This was achieved by injecting an aliquot of solution containing CO and simultaneously changing the gas composition reaching the headspace by replacing, with CO, a certain fraction of the 20% N2 component of the 80% H2 mixture. Exemplary results obtained with CaHydA are shown in Figure 5. Panel A shows the time-course for CO inhibition of H2 oxidation at -0.05 V, and Panel B shows the same time-course for inhibition of H+ reduction at -0.4 V. Both experiments commenced with the hydrogenase on the electrode being exposed to an atmosphere of 80% H2, 20% N2. At t ) 300 s, an aliquot of CO-containing solution was injected to take the cell concentration to 2% CO, and the cell was flushed with 80% H2, 18% N2, 2% CO. Once the catalytic current had decreased to a steady level, the CO was removed from the cell by flushing with 80% H2, 20% N2. (Note that 80% H2 remained in the headspace of the cell throughout the experiment. This excludes variations in catalytic current resulting simply from changes on H2 concentration, which would otherwise pose a problem, given the high KM of these enzymes as indicated in Panel 1.) Pseudo first-order rate constants (k′inact) were determined for each experiment from the slope of plots of ln(i) vs t, with linearity generally exceeding 75% for H2 oxidation, although less so for H2 production. Panels C and D show the corresponding dependencies of k′inact on CO concentration, obtained for DdHydAB, CrHydA1 and CaHydA by numerous experiments conducted analogously to those shown in panels A and B. Values for the equilibrium inhibition constant KICO(equil) were determined for CrHydA1 and CaHydA by measuring the H2 oxidation current stabilized at different concentrations of CO. 14984

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The term KICO(equil) depends on H2 concentration and potential, but we kept these variables constant (80% H2, -0.05 V). These titrations were similar in design to those shown in Figure 2C and are described in more detail in Supporting Information (Figure SI.3). Values of KICO(equil) are shown in Table 2. Comparable data for DdHydAB could not be obtained because the enzyme was fully inhibited at the lowest practical CO concentrations. Panel B of Figure 5 shows that CO inhibition of H2 production of these enzymes is only partially reversible. This observation was consistently made with CaHydA and CrHydA1 but the instability of DdHydAB films prevented us from making similar measurements with this enzyme. By comparison, CO inhibition is fully reversible when measuring H2 oxidation at -0.05 V. In all cases, the background decrease in current that we refer to as film loss (traced by the dashed line) was checked through control experiments carried out without introducing CO. The rates determined for the inactivation of H2 production by CO are therefore approximate. The potential dependence of the rate of inactivation of H2 production was not investigated further, although we noted that the inactivation process became noticeably biphasic as the potential was lowered below -0.4 V. We raise this issue later in the Discussion. Panels C and D of Figure 5 show that: (i) DdHydAB is always the fastest to react with CO and CaHydA the slowest, and ii) for all three enzymes, inhibition of H2 oxidation measured at -0.05 V is substantially faster than inhibition of H2 production at -0.4 V, but the same order DdHydAB > CrHydA1 > CaHydA is observed in both catalytic directions. The secondorder rate constants (kinact) are provided in Table 2 (see later). In all cases, the reactivation rates (kre-act) are the same, within reasonable error, for H2 oxidation and H2 production. Experiments carried out with CrHydA1 and CaHydA showed that kre-act is strongly light sensitive (see Supporting Information, Figure SI.1) in agreement with earlier electrochemical observations made with DdHydAB.47 In addition, kre-act increased significantly when the temperature was raised to 25 °C. From the ratio of rate constants for the reactivation and inactivation reactions at 10 °C, we derived the kinetic inhibition constants KICO(kin) ) kre-act/kinact. In Table 2, KICO(kin) values are compared with the equilibrium values, KICO(equil), obtained by titration for CrHydA1 and CaHydA. The two values for CrHydA1 are in good agreement, although for CaHydA, KICO(equil) is rather higher than KICO(kin). The ratio of the kinact values for CO inhibition of H2 oxidation and H2 productionsthe “catalytic direction discrimination”- is about 49 for DdHydAB, 22 for CrHydA1 and 3 for CaHydA: these variations clearly arise from differences in the rate that CO binds because kre-act is similar for all three enzymes. Reactions with O2. Figure 6 shows experiments in which each enzyme is subjected to 5% O2 during H2 oxidation at -0.05 V vs SHE. As before, the H2 concentration was kept constant throughout the entire time-course. Each experiment began with 80% H2, 20% N2 flushing through the cell headspace. The gas mixture was then switched to 80% H2, 15% N2, 5% O2 for the duration of the reaction (this involved simultaneous injection of an aliquot of gas-equilibrated buffer for DdHydAB and CrHydA1; the kinetics of O2 inactivation of CaHydA were so slow that the injection was unnecessary). The gas was finally changed back to 80% H2, 20% N2 once most of the activity (>90%) had been eliminated. We noted that the rate of inactivation by O2 showed a dependence on H2 concentration, with inactivation occurring more rapidly at lower levels, thus

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Table 2. Compilation of kinact and kre-act Values for CO Inhibition of H2 Production and H2 Oxidation for DdHydAB, CrHydA1 and CaHydA,

with the Corresponding Catalytic Direction Discrimination Factors and Values of KICO Determined by Kinetic and Equilibrium Methodsa DdHydAB -1

-1

CaHydA

H2 oxidation kinact/s µM

3.9 × 10 (3 × 10-1

-2

1.9 × 10 (1 × 10-2

1.1 × 10-3 (7 × 10-4

H2 production kinact/s-1µM-1

8.0 × 10-3 (7 × 10-3

8.4 × 10-4 (2 × 10-4

3.6 × 10-4 (1 × 10-4

49 1.9 × 10-3 (2 × 10-4

22 2.3 × 10-3 (1 × 10-3

3 1.8 × 10-3 (1 × 10-3

2.7 × 10-3 (1 × 10-3

1.2× 10-3 (3 × 10-3

2 × 10-3 (2 × 10-4

-

1.0 × 10-1

2.2 × 10-1

KICO (kin)/µM for H2 oxidation (at -0.05 V)

4.8 × 10-3

1.2 × 10-1

1.6

KICO (kin)/µM for H2 production (at -0.40 V)

0.34

1.4

5.6

Catalytic direction discrimination H2 oxidation kre-act/s-1 H2 production kre-act/s-1 KICO (equil)/µM for H2 oxidation (at -0.05 V)

a

CrHydA1

-1

All values pertaining to H2 production were measured at -0.40 V.

Figure 7. Dependence of rate constants (k′inact) for inactivation of enzymatic H2 oxidation by O2, on O2 concentration, for DdHydAB, CrHydA1 and CaHydA (shown as an expanded scale). Experimental conditions: pH 6.0, 10 °C, electrode rotation 3000 rpm, -0.05 V vs SHE, headspace gas mixture composed of 80% H2, 20% mixture of N2 and O2.

Figure 6. Inactivation of H2 oxidation by DdHydAB, CrHydA1 and

CaHydA by 5% O2. The headspace mixture is composed of 80% H2 and the remaining 20% is as indicated. For DdHydAB and CrHydA1 a 0.67 mL aliquot of 20% O2/80% H2-saturated buffer was injected into the cell which initially contained 2 mL of buffer, to give an instant concentration of 5% O2 at the points marked by the dashed arrows. For CaHydA, the reaction was sufficiently slow that it could be initiated simply by changing the headgas mixture. Other conditions: pH 6.0, 10 °C, electrode rotation 3000 rpm, -0.05 V.

indicating that O2 and H2 are competitive.34 We also noted that after removing O2 from the cell, a very small amount of activity was consistently recovered for all three enzymes. Oxygen

undergoes very slow reduction at graphite at -0.05 V, therefore control experiments (Figure SI.4, Supporting Information) were performed to assess the contribution to the current from O2 reduction. This contribution to the current was then subtracted from the experiments to verify the end point and the activity remaining. In separate anaerobic experiments we determined that small quantities of hydrogen peroxide that would be formed during electrodic O2 reduction did not cause inactivation, by injecting aliquots of H2O2 solution and monitoring the effect on the H2 oxidation current at -0.05 V (10 °C). This decrease in current was observed to be much slower than that due to equivalent concentrations of O2 for all three enzymes. Figure 7 shows how rate constants for O2-inactivation of H2 oxidation activity vary with O2 concentration for all three [FeFe]-hydrogenases. In each case the rate of inactivation is first-order in O2 concentration. Table 3 shows the rate constants for inactivation by O2 alongside the rate constants for inhibition of H2 oxidation by CO. For each enzyme, the rate of reaction with CO is much faster (80-200-fold) than with O2 and this ratio, which we refer to as the “gas identity discrimination”, is also included in Table 3. J. AM. CHEM. SOC.

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Table 3. Comparison of the Second-Order Rate Constants for

Inhibition by CO (kinact(CO)) and Inactivation by O2 (kinact(O2)) of DdHydAB, CrHydA1 and CaHydA and Evaluation of the Gas Identity Discriminationsthe Ratio kinact(CO)/kinact(O2) enzyme

DdHydAB CrHydA1 CaHydA

kinact(CO)/s-1µM-1 -1

3.9 × 10 1.9 × 10-2 1.1 × 10-3

kinact(O2)/s-1µM-1

kinact(CO)/kinact(O2)

1.8 × 10-3 ( 3 × 10-4 2.2 × 10-4 ( 1 × 10-4 5.1 × 10-6 ( 1 × 10-6

217 86 216

H2 Production in the Presence of O2. Finally, we carried out experiments to estimate the extent to which the rate of inactivation by O2 depends on whether the hydrogenase is operating in the direction of H2 production or H2 oxidation. We used a procedure described recently,44 in which the problem of distinguishing the current due to enzymatic H+ reduction from that due to electrochemical O2 reduction is resolved by adding an inhibitor. The decrease in current observed when the inhibitor is added provides a direct measure of the component of the current due to enzyme-catalyzed H+ reduction. The experiments conducted for DdHydAB, CrHydA1 and CaHydA are shown in Figure 8. The experiment performed on DdHydAB started under an atmosphere of N2, and a current corresponding to enzymecatalyzed H2 production was recorded (stage 1). The headgas was then switched to 1% CO in N2, and a rapid and almost complete loss of current was observed. When the CO was flushed from the cell, the current increased and reached a steady level (albeit not the same level as prior to CO introduction, due to the partial irreversibility of this reaction, see Figure 4). The current that was recovered during CO efflux was adopted as the normalization unit for the next stage. In stage 2, O2 was introduced: the current initially increased due to direct reduction of O2 at the graphite electrode but then began to decrease as the enzyme became inactivated. Introduction of 1% CO after 2000 s under O2 resulted in a rapid loss of current, the magnitude of which reports on the enzyme-catalyzed current component prior to CO inhibition. Removal of CO caused the current to increase again, but a further decrease in current was observed, as expected, when O2 was removed from the cell after t ) 8500 s. Finally in stage 3 the inhibition step with CO was repeated to establish the extent of survival of the hydrogenase. Similar sequences of steps were used in the experiments with CrHydA1 and CaHydA, except that 99% and 95% CO, respectively, were used instead of 1% CO in order to compensate for the much slower kinetics and lower CO affinity of these hydrogenases compared to DdHydAB; in addition, 5% O2 was used to obtain a higher rate of inactivation for CaHydA. To estimate the half-life for inactivation in each case, the decrease in current upon addition of CO after reaction with O2 for t seconds (during stage 2) was divided by the original increase in current observed when CO was removed during stage 1. The half-life was calculated using t1/2 ) -t ln 2/ln(x) where x is the fraction of H2 production current surviving after t seconds. From control experiments such as those previously described,44 it was estimated that the concentrations of O2 experienced by the enzyme at -0.4 V under headgas conditions of 1% and 5% were 0.7% and 4.5%, respectively. The corresponding half-lives for H2 oxidation activity under similar O2 concentrations were calculated from the rate constants in Table 3. The results and comparisons are shown in Table 4. All enzymes remained at least 30% active after 2000 s under 1% bulk O2 (i.e., at least 0.7% O2 at the electrode44) for DdHydAB and CrHydA1 and even under 5% bulk O2 14986

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Figure 8. Chronoamperometric experiments designed to measure the survival of H2 production activity in the presence of headspace levels of 1% O2 for DdHydAB and CrHydA1, and 5% O2 for CaHydA (note change in current scale upon introduction of 5% O2). All procedures were carried out using 1 bar pressure of the gases indicated. Other conditions: pH 6.0, 10 °C, electrode rotation 3000 rpm, -0.4 V vs SHE. The blue vertical lines show introductions of CO to inhibit and reveal enzyme-catalyzed H2 production. The fraction of enzyme surviving O2 after time t is given by the ratio of CO-sensitive current measured in Stage 2 relative to that measured after the recovery in Stage 1 (indicated by vertical double-headed arrows in each case). Table 4. Comparative Half-Lives (s) for O2-Inactivation of H2

Production and H2 Oxidation of DdHydAB, CrHydA1 and CaHydAa catalytic direction

DdHydAB

CrHydA1

CaHydA

H2 oxidation H2 production Catalytic direction discrimination

30 ∼2000 60-70

305 ∼1320 4-5

1890 ∼2250 approximately 1

a The half-lives for the H2 production reaction are estimated on the basis of the percentage of activity remaining after an extended period (1800-2000 s, different for each enzyme) of catalytic turnover in the presence of O2. Experimental conditions for H2 production: DdHydAB, CrHydA1; 1% O2 in the headgas (i.e. at least 0.7% O2 surviving at the electrode); CaHydA, 5% O2 in the headgas (at least 4.5% surviving at the electrode) -0.4 V, pH 6, 10 °C, electrode rotation rate 3000 rpm. Data for H2 oxidation were calculated using the second-order rate constants (see Table 3) and values of 0.7% O2 for DdHydAB and CrHydA1; and 4.5% O2 for CaHydA.

(around 4.5% O2 at the electrode) for CaHydA. More significantly, there were important differences among the three enzymes when comparing their survival to O2 exposure

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during H2 production with the data obtained for H2 oxidation. In the case of DdHydAB, the H2-production activity remaining is about sixty-fold greater than expected on the basis of the results described above for O2 inactivation of H2 oxidation. The enhancement is also observed for CrHydA1, but to a lesser extent than DdHydAB. On the other hand, no clear difference was observed for CaHydA between the rates of inactivation observed when monitoring H2 production at -0.4 V or H2 oxidation at -0.05 V. Discussion

The three hydrogenases we have investigated include two with potential applications in large-scale H2 production (CrHydA1 for photosynthesis and CaHydA for fermentation) and one of known crystal structure (DdHydAB).48 In addition, CaHydA is closely related to CpI for which the structure is known.22,23 Some important comparisons have been made, exploiting the unique ability of protein film electrochemistry to measure, simultaneously, the rates and extent of changes in catalytic activities under well-defined potentials (driving force). A summary of the quantitative observations and interrelationships is provided in Figure 9. The reactions with CO are highly informative, and we34 and others39 have noted that CO protects [FeFe]-hydrogenases against O2 degradation, suggesting both inhibitors target the same site. In all cases we could use CO as a strong inhibitor of both H2 oxidation and H2 production, helped by the fact that binding of H2 under both conditions is much weaker than CO binding49 (see Table 1). Light sensitivity of CO inhibition is a well-established property of [FeFe]-hydrogenases13,30,33,47,50-53 and originates from the photolability of the Fe-CO bond.54 A particularly useful result is the similarity in the rates of dark reactivation of the CO-inhibited hydrogenases. In all cases, the rate is accelerated by illumination (as reported in an earlier study for DdHydAB47), and this suggests strongly that the reaction being observed in all cases is an elementary dissociation of the Fe-CO bond. This result demonstrates an intrinsic property of the H-cluster, maintained regardless of the slightly differing protein environments among the [FeFe]-hydrogenases. Evidently, all that is required for reactivation is to liberate the coordination site and ensure that CO escapes from the pocket before it can recombine.30 Our KICO data for CaHydA determined during H2 oxidation lie broadly in the range of values (around 1 µM) obtained by Thauer and co-workers50 for CO binding to the related enzyme from C. pasteurianum, although those experiments also used a higher temperature and we found a consistently higher value (48) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps, J. C. Struct. Folding Des. 1999, 7, 13–23. (49) This allowed for the same concentration of H2 to be employed in the experiments at -0.4 and -0.05 V, thus ensuring that the experiments were comparable. For other hydrogenases, strong H2 inhibition of H2 production at -0.4 V would have prevented experiments performed at this potential from being performed under 80% H2 as they were at -0.05 V. (50) Thauer, R. K.; Kaufer, B.; Zahringe, M.; Jungerma, K. Eur. J. Biochem. 1974, 42, 447–452. (51) Purec, L.; Krasna, A. I.; Rittenberg, D. Biochemistry 1962, 1, 270– 275. (52) Kempner, W.; Kubowitz, F. Biochem. Z. 1933, 257, 245. (53) Albracht, S. P. J.; Roseboom, W.; Hatchikian, E. C. J. Biol. Inorg. Chem. 2006, 11, 88–101. (54) Kochanski, E. Photoprocesses in Transition Metal Complexes, Biosystems and Other Molecules: Experimental and Theory; Kluwer Academic Press: London, 1992.

Figure 9. Bar charts representing various comparisons between DdHydAB, CrHydA1, and CaHydA. (A) Comparative rates of CO-inhibition of H2 oxidation (kinact(CO/H2), H+ reduction (kinact(CO/H+) and rates of recovery from CO-inhibition (kre-act(CO)). (B) Discrimination factors characterizing the favorability of binding CO over O2 (kinact(CO/H2)/kinact(O2/H2)), binding O2 when the enzyme is catalyzing H2 oxidation compared to H2 production (kinact(O2/H2)/kinact(O2/H+)) and binding CO when the enzyme is catalyzing H2 oxidation compared to H2 production (kinact(CO/H2)/kinact(CO/H+)) for DdHydAB, CrHydA1 and CaHydA. As the rate of reactivation from CO inhibition is essentially independent of the process being catalyzed, it is simply represented by the term kre-act(CO). The kinact(O2/H2)/ kinact(O2/H+) ratios are approximate because the values of kinact(O2/H+) are estimates. This ratio is approximated to 1 for CaHydA.

(weaker binding) for the kinetic compared to the equilibrium value. The reasons for this are unclear at present, and doubtless the model we now discuss is oversimplified. The rate data for all three enzymes, both qualitative (Figure 3) and quantitative (Figure 4 and Figure 5), reveal a preference for CO binding to the enzymes under H2 oxidationsthe direction enforced by a higher electrode potential that should ensure that Hox predominates over Hred during the catalytic cycle. The “catalytic direction discrimination” for CO decreases in the order DdHydAB > CrHydA1 . CaHydAsthe same order as found for the equivalent factor estimated for O2 inactivation. The order also matches that observed for the rates of inactivation of H2 oxidation by CO and O2. These interrelationships lead us to propose a model for the reVersible binding of CO to [FeFe]hydrogenases which can be extended to account for reactions with O2. The model is represented schematically in Figure 10A. The model considers that the attack by CO (rate constant kinact) involves two stages: the first stage is transport of CO (we treat this generically as the small molecule X) from the external medium (Xext) through the enzyme (for simplicity we show this as a single tunnel without branches or sites at which X could J. AM. CHEM. SOC.

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Recognizing this is a simplistic model, we now consider the following limiting scenarios: (i) if kout e k2, k′inact ∼ kin[Xext] so the rate of inhibitor binding depends only on the external concentration and rate of internal transport of X; in this case little discrimination is expected based on the redox state of the H-cluster. Alternatively, (ii), if kout . k2, i.e. if the protein’s internal structure does not provide an effective barrier to transport of X, it follows that k′inact ) k2kin[Xext]/kout. In this scenario, the rate of inhibition depends not only on the nature of X but also upon k2 and therefore should also be faster for conditions favoring Hox (H2 oxidation) compared to Hred (H2 production). Reactivation follows the reverse sequence, and for kin[Xext] ) 0 (because CO is removed from the solution) we obtain rate of re-activation )

koutk-2[X-Fed] kout + k2

(4a)

and the first-order rate constant (as measured experimentally) is given by Figure 10. (A) Cartoon depicting the stepwise reaction of an inhibitory gas molecule X () CO) with the buried H-cluster (Fed) of a [FeFe]hydrogenase, showing transfer from the external medium (Xext) to a position (Xint) close to Fed (kin, kout), and inner-sphere coordination/dissociation at Fed (k2, k2). (B) C-R tracing of the [FeFe]-hydrogenase from D. desulfuricans: DdHydAB(large subunit: blue and green, small subunit: red) and the cavity observed in the crystals (probe size: 0.8 Å, program CAVsel, A. Volbeda, unpublished). The H-cluster occupies the center of the molecule. The blue sphere represents the experimentally observed Xe site. Other color codes: red: Fe, yellow: S.

be trapped) to a noncoordinating site close to the H-cluster; the second stage is migration of Xint from the noncoordinating site to a coordination site that we assume to be Fed (the actual innersphere binding reaction). The process of reactivation (overall rate constant kre-act) is the reverse of this reaction scheme. The terms kin and kout are rates of transport of X through the protein in either direction, and k2 and k-2 are the elementary rates of binding and dissociation of ligand X, respectively, to Fed within the region of the active-site pocket. With the simplification that k-2[X-Fed] is negligible until the reaction of inhibitor with enzyme is essentially complete, the steady-state approximation with d[Xint]/dt ) 0 yields: rate of inactivation )

kink2[Xext][Fed] kout + k2

(3a)

where the pseudo first-order rate constant (as measured directly in experiments) is: kinact ′ )

kink2[Xext] kout + k2

(3b)

An assumption of this model is that kin and kout should depend on the nature of the gas molecule and gas filter but not on the catalytic state of the H-clustersthe state predominating for a particular electrode potential, i.e. Hox or Hred. Based on the evidence that CO binds preferentially (and perhaps exclusively) to Hox,29 we expect that k2 for CO will be large for Hox and small, even zero, for Hred. Note however that inhibition is still observed during H2 production because Hox may always appear briefly in the catalytic cycle, even at the lowest potentials we have used (-600 mV in the cyclic voltammetry experiments). 14988

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kre-act )

koutk-2 kout + k2

(4b)

Under the limiting condition kout , k2, kre-act ) koutk-2/k2, whereas if kout . k2, the rate of reactivation reduces to k-2, reflecting the likelihood that CO escapes from the enzyme (kout) before it can recoordinate (k2). Our data suggest that the latter situation must generally be the case, with a more intermediate situation (a smaller kout relative to k2) applying for CaHydA (see below). Overall, the dissociation constant is given by KICO(kin) ) kre-act/kinact ) koutk-2/kink2, which always depends on the kinetics of making and breaking the Fe-CO bond. This analysis can be extended to the reaction of [FeFe]hydrogenases with O2, although that reaction is essentially irreversible. Table 3 and Figure 9 show that trends among the hydrogenases as observed for their reactions with CO are mirrored in their reactions with O2; for example, DdHydAB shows the highest rates of inhibition in both cases and the greatest discrimination based on catalytic direction. In mechanistic terms, the minuscule protection that DdHydAB possesses against attack by O2 is provided only within the activesite pocket in which O2 is able to discriminate between different catalytic states of the enzyme (a strong k2 dependence, according to the model). In contrast, the small catalytic direction discrimination observed for CaHydA can be interpreted in terms of it showing a less excessive value of kout (a more restrictive tunnel or filter) in relation to k2. Values for kout should correlate closely with those for kin;thus, it is significant that CaHydA also shows the slowest rates of reaction with CO and O2 and, with a halflife of several minutes under atmospheric O2 levels at 10 °C, looks to be a promising model for aerobic biohydrogen production even though it stems from a strict anaerobe. The evidence (strong light enhancement) that the ratedetermining step in reactivation is the elementary scission of the Fe-CO bond in the Hox-CO state, and the observation that the rate measured in the dark is quite similar for all enzymes (which share only 40% sequence similarity) shows that the kinetics of reactivation are governed more by the intrinsic properties of the H-cluster than by the nature of the surrounding enzyme. Lubitz and colleagues have proposed that the H-clusters in CrHydA1 and DdHydAB are similar, although they differ slightly in electronic detail.28 Note that were CO to coordinate

O2 Inactivation Kinetics of Three [FeFe]-Hydrogenases

to different states of the H-cluster at -0.4 and -0.05 V, we would expect kre-act to depend significantly on potential, but it does not. This supports the view that CO (and by extension, O2) binds to Hox but not Hred. The other comparison in Figure 9 which is reasonably constant among all three enzymes is the gas identity discrimination (CO vs O2). This again may reflect intrinsic behavior of the H-cluster because CO is a superior ligand to O2 in terms of its π-acceptor capability. Dominant intrinsic effects are not unexpected, given the unusually independent status of the 2FeH subcluster, which was described in the Introduction as an enzyme cofactor resembling an organometallic compound buried in a protein. Clearly our model is an oversimplification, albeit necessary at this stage, and to understand this observation more fully we are undertaking theoretical calculations, including predictions of relative transport rates through the enzyme. The notion of a filter or a tunnel connecting the H-cluster to the molecular surface is supported by the two available [FeFe]hydrogenase structures. In DdHydAB and CpI there is a ‘static’ tunnel that can be revealed using a cavity-searching program.55 The tunnel in each enzyme has a central cavity that can bind a Xe atom and a narrower path leading to Fed.17,26 Figure 10B depicts the experimentally observed tunnel and Xe site in DdHydAB.56,57 The tunnel connects the molecular surface to the active site, and one possibility is that the Xe atom occupies a cavity in which dissociated CO could reside before rebinding to Fed or escaping to the medium. Molecular dynamics simulations based upon the CpI structure revealed a second tunnel that also connects to the central cavity.58 What can be concluded from both crystallographic and theoretical studies is that dynamic fluctuations are important for intramolecular gas diffusion in [FeFe]-hydrogenases. Two further mechanistic points emerge from this study. First, we always observed that CO inhibition of H2 production is only partially reversible. At present we have no explanation for this, although Adams reported in 1987 that CO binds irreversibly to a catalytic intermediate of CpI.59 Further investigations including a full study of the potential dependence of CO binding during H2 production are clearly required to resolve this issue, which may have mechanistic relevance. Second, we always recorded a small proportion of activity returning after O2 inactivation, an observation in line with those reported by Baffert et al.39 (55) Montet, Y.; Amara, P.; Volbeda, A.; Vernede, X.; Hatchikian, E. C.; Field, M. J.; Frey, M.; Fontecilla-Camps, J. C. Nat. Struct. Biol. 1997, 4, 523–526. (56) Fontecilla-Camps, J. C.; Amara, P.; Cavazza, C.; Nicolet, Y.; Volbeda, A. Nature 2009, 460, 814–822. (57) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13–23. (58) Cohen, J.; Kim, K.; Posewitz, M.; Ghirardi, M. L.; Schulten, K.; Seibert, M.; King, P. Biochem. Soc. Trans. 2005, 33, 80–82. (59) Adams, M. W. W. J. Biol. Chem. 1987, 262, 15054–15061.

ARTICLES

also in studies on CaHydA. Such a part-reversal is consistent with the mechanism proposed by Stripp et al.34 in which O2 must first bind in a reversible manner at the distal Fe of the 2FeH subcluster before causing irreversible damage to the [4Fe-4S]H subcluster. From a biological perspective, the precise, quantitative data that we have been able to extract and compare for the three different hydrogenases should be understandable in terms of the lifestyles of the organisms that express them. This is true, in part. Recent studies on the O2 detoxification mechanism in C. acetobutylicum have shown that this fermentative bacterium can survive limited exposure to air and can even undergo cell division at surprisingly high concentrations of O2.60,61 The relative O2 stability of CaHydA may therefore be a consequence of concerted evolutionary adaptation to an O2-rich atmosphere. However, certain species of the DesulfoVibrio genus have also been reported to exhibit short-term survival when exposed to O2;62,63 thus, from a microbiological viewpoint the large disparity in O2 sensitivity between CaHydA and DdHydAB is puzzling. The ‘intermediate’ degree of O2 sensitivity displayed by CrHydA1 is consistent with the observation that although it is only expressed in C. reinhardtii under anaerobiosis64,65 it is likely to be in contact with at least trace amounts of O2 that are produced by photosystem II.66 Acknowledgment. Research in the group of FAA was supported by the UK BBSRC (Grant BB/D52222X) and EPSRC. C.C. and J.C.F.C. thank the CEA and the CNRS (France) for institutional funding. S.S. and T.H. were supported by the Deutsche Forschungsgemeinschaft (SFB 480) and EU/Energy Network SolarH2 (FP7 418 contract 212508. We thank Dr. Alison Parkin for providing part of Figure 3 and for helpful advice, and Dr. A. Volbeda for preparing Figure 10B. Supporting Information Available: Photolability of the CObound state; determination of the variation of the concentration of CO with time in the experiments shown in Figure 4; determination of KICO(equil); control experiments for experiments investigating O2 inactivation of H2 oxidation. This material is available free of charge via the Internet at http:// pubs.acs.org. JA905388J (60) Hillmann, F.; Fischer, R. J.; Saint-Prix, F.; Girbal, L.; Bahl, H. Mol. Microbiol. 2008, 68, 848–860. (61) May, A.; Hillmann, F.; Riebe, O.; Fischer, R. M.; Bahl, H. FEMS Microbiol. Lett. 2004, 238, 249–254. (62) Cypionka, H. Annu. ReV. Microbiol. 2000, 54, 827–848. (63) Lumppio, H. L.; Shenvi, N. V.; Summers, A. O.; Voordouw, G.; Kurtz, D. M. J. Bacteriol. 2001, 183, 2970–2970. (64) Happe, T.; Naber, J. D. Eur. J. Biochem. 1993, 214, 475–481. (65) Happe, T.; Kaminski, A. Eur. J. Biochem. 2002, 269, 1022–1032. (66) Happe, T.; Hemschemeier, A.; Winkler, M.; Kaminski, A. Trends Plant Sci. 2002, 7, 246–250.

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